1. Technical Field
Disclosed herein are methods and apparatuses for controlling the opening and closing of combustion cylinder intake valves of dual-fuel internal combustion engines.
2. Description of the Related Art
Technology that allows diesel engines to run primarily on liquefied natural gas (LNG) may provide an economical way to shift fuel consumption from diesel to LNG. Such a shift could lower greenhouse gas emissions, because burning LNG emits less carbon dioxide per unit of energy than diesel. Shifting from diesel to LNG may save costs, because LNG is cheaper per unit of energy than diesel. Dual-fuel engines, which burn both diesel and LNG, are advantageous because an operator can revert to diesel if LNG is not available or if the price of natural gas rises.
Diesel engines may be converted to run primarily on LNG with relatively small modifications. In a diesel engine, the air-fuel mixture is not ignited with a spark, as in gasoline engines, but by compressing the air until the air-fuel mixture gets hot enough to combust. Compression ignition, as the process is called, does not work well with LNG alone because it is too difficult to control exactly when combustion occurs. As a result, the LNG can detonate and damage the engine. In a dual-fuel engine, this problem is solved by injecting a small amount of diesel into the cylinders with the LNG to trigger combustion. Hence, diesel engines converted to burn LNG also burn small amounts of diesel.
Diesel engines are typically four-stroke engines the operate with a diesel cycle. In an ideal diesel cycle, constant pressure during the combustion is assumed. In contrast, in an “Otto cycle,” constant volume during the combustion is assumed. During an Otto cycle, the piston completes four separate strokes to complete a single thermodynamic cycle. A stroke refers to the full travel of the piston along the cylinder, in either direction. The four strokes are (i) intake, (ii) compression, (iii) power and (iv) exhaust. The intake stroke of the piston begins at top of the cylinder with the intake valve(s) open and the exhaust valve(s) closed. The piston descends from the top to the bottom of the cylinder as air is injected into the cylinder. During the compression stroke, both the intake and exhaust valves are closed and the piston returns towards the top of the cylinder, compressing the air. At the beginning of the power stroke, diesel fuel is injected into the cylinder and the piston is close to the top of the cylinder and the compressed hot air ignites the diesel fuel with the heat generated by compressing the air. The resulting pressure from the combustion forces the piston back towards bottom of the cylinder to complete the power stroke. Finally, during the exhaust stroke, the piston once again returns to top of the cylinder while the exhaust valve(s) are open and to expel the spent exhaust gases from the cylinder.
A Miller cycle is a variation of the Otto cycle. During a traditional Miller cycle, the intake valve remains open for part of the compression stroke. In effect, the compression stroke is divided into two discrete portions or stages: the initial portion, when the intake valve(s) remain open and final portion when the intake valve(s) are closed. To counteract the loss of power resulting from the shorter compression stroke and reduced compression ratio, Miller engines are equipped with a supercharger. Pressurized air from the supercharger passes through an intercooler, which lowers the air temperature of the air, making it denser, so more air fits within the same volume during the intake stroke. During a Miller intake stroke, a charge of cold, compressed air rushes into the cylinder with the intake valve(s) open, filling the cylinder with more air than in an Otto cycle intake stroke. As the compression stroke starts with the intake valve(s) open, the output of the supercharger keeps the cylinder pressurized until the intake valve(s) close, thereby limiting the amount of air that is pushed out of the cylinder and into the intake manifold. Further, compressing the air against the pressure from the supercharger requires less energy than compressing the air mixture in a closed cylinder, thereby reducing pumping losses compared to a traditional Otto engine.
The compression ratio of an Otto cycle is higher than that of a Miller cycle because an Otto cycle has a longer effective compression stroke than a Miller cycle. While a high compression ratio is desired for diesel, use of an Otto cycle and a high compression ratio with LNG may result in knocking. A lower effective compression ratio is preferred for LNG than for diesel, which will allow for maximum gas substitution. As a result, an Otto cycle is preferred for diesel and a Miller cycle is preferred for LNG. However, dual-fuel engines need to be able to convert from burning diesel to burning LNG and vice versa. For example, dual-fuel engines may need to switch back to burning diesel if the LNG supply runs out by returning to an Otto cycle or a diesel cycle. Further, it may be easier to start an engine that runs on diesel. To switch between an Otto cycle and a Miller cycle or to change the compression ratio, the timing of the intake valves must be changed. Thus, for dual-fueled engines, the timing of the intake valve closing must be varied to change the compression ratio or to switch between an Otto cycle and a Miller cycle or a variation thereof.
Therefore, there is a need for controlling the closure of the intake valves of an internal combustion engine to enable the engine to run on diesel alone in a diesel cycle or an Otto cycle or to run on diesel in combination with LNG in a Miller or Miller-like cycle.